1. Field of the Invention
The field of the invention is that of optical velocimetric probes for defining the velocity of particles in relative motion with respect to the probe.
2. Discussion of the Background
There are currently various optical devices for optically measuring the velocity of particles having small dimensions (typically less than one micron).
For this, use may be made of probes operating on the principle of the longitudinal Doppler effect. A light source sends a beam onto a particle, and the back-scattered light is analysed; the velocity of this particle gives the back-scattered energy a variation in frequency with respect to the frequency of the incident radiation. In order to obtain a significant measurement, it is essential to provide a source whose wavelength is perfectly known, in order to determine a precise variation with respect to the said wavelength.
Other optical systems currently known comprise means for making two incident rays interfere in a measurement volume through which particles whose velocity is to be measured pass. The energy back-scattered by the particles carries an indication of the velocity. The benefit of interferometry resides in the fact that it is not necessary to provide a perfectly monochromatic source. By way of example, FIG. 1 illustrates a device which uses this type of interference.
A source 11 of the diode laser type emits a light beam L.sub.1 in the direction of an optical splitter 12 which can generate two light beams L.sub.11 and L.sub.12 in the direction of a return optical system MR, so that the two beams L.sub.11 and L.sub.12 interfere in the measurement volume MV, by means of a lens 13, and thereby create interference fringes which are crossed by a particle P passing through the measurement volume in the direction DZ indicated in FIG. 1. The said particle back-scatters light, in the form of the beam L.sub.2, and this light is collimated through the lens 13 then focused through a lens 14 in the direction of photodetection means 15. The means are coupled to a signal processing device 16 which can extract information relating to the velocity of the particle on the basis of the electrical signal delivered by the photodetection means 15.
The major problem of this type of conventional fringe probe resides in the weak energy back-scattered by submicron particles, this energy being poorly suited to the sensitivity of conventional photodetectors.